Fix incorrect error handling of call to mbrtowc. This is PR#13759. Leaving the bug open because (1) I'm not sure that we're correct here, only better than before, and (2) no tests

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@233012 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed